The present invention relates to an X-ray permeable membrane used for an X-ray lithographic mask. More particularly, it relates to an X-ray permeable membrane used for an X-ray lithographic mask which has excellent transmissivity to visible light, resistance against high energy beam irradiation, resistance against chemicals and resistance against moisture, and is smooth and free from pin holes.
Along with the trend toward fine pattern formation in semiconductor devices, X-ray lithographic technology is considered to be the most promising lithographic technology for the future. The X-ray lithographic mask consists of an X-ray permeable membrane with an X-ray absorbing body formed on the surface thereof in a desired shape, and reinforced with a substrate body. The X-ray permeable membrane is also called an X-ray support (membrane) or membrane for X-ray permeation supporting an X-ray absorbing body.
Generally speaking, an X-ray permeable membrane has a thin film of 10 .mu.m or smaller thickness, made of an inorganic material consisting of a light element which has a small X-ray absorption coefficient. The x-ray permeable membrane also has an X-ray absorbing body, composed of an inorganic material consisting of a heavy element having a large X-ray absorption coefficient such as gold (Au), tungsten (W), and tantalum (Ta).
X-ray permeable membranes in general are required to have the following properties.
(i) To be made from a material capable of withstanding irradiation with high-energy electron beams and light energy beams such as synchrotron radiation (SOR light). PA0 (ii) To have transmissivity to visible light suitable for high precision alignment (position-matching) work. PA0 (iii) To be highly resistant to chemicals and moisture. PA0 (iv) To be resistant to damage which may occur during the etching and washing process. PA0 (v) To have a smooth membrane surface and to be free from warping, creases, defects or pin holes.
The base materials proposed in the prior art for an X-ray permeable membrane of X-ray lithographic mask include materials such as boron nitride (BN), silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC) and the like. While BN has superior transmissivity to visible light, BN is vulnerable to high-energy beams. Similarly, Si.sub.3 N.sub.4 is vulnerable to chemicals and moisture, and SiC has inferior transmissivity to visible light, yet each have advantages. Nothing has been discovered heretofore which satisfies all of the required properties mentioned above.
Further, while the X-ray permeable membranes of BN, Si.sub.3 N.sub.4, SiC and the like are usually prepared by the chemical vapor-phase deposition method (CVD), this CVD method is accompanied by chemical reactions and decomposition of the gaseous starting material so that other elements and constituents are sometimes taken into the X-ray permeable membrane as impurities. Consequently, this results in the removal of the impurities or, in particular, hydrogen in the membrane by irradiation with SOR light energy beams. This causes distortion of the membrane, variation in stress, decrease in the mechanical strengths, decrease in the optical transparency, and the like. The problem in finding satisfactory membranes can hardly be avoided because pin holes and nodules are sometimes formed on the surface of the membrane.
It is said that, among these materials, SiC, having a particularly high Young's modulus, may be the most advantageous material of the membrane with respect to resistance to high-energy.